Yoke driving circuit which reduces transients



p 3, 1966 c. A. TOMASZEWSKI 3,273,009

YOKE DRIVING CIRCUIT WHICH REDUCES TRANSIENTS Filed Jan. 2, 1964 PLATESUPPLY YOKE 2 l INPUT MONITORING CIRCUIT I I PLATE I CURRENT I P TRIODEI OPERATION PLATE VOLTAGE Ep cuRRENT J I p TETRODE 0R PENTODE oPERATIoNI P I COMBINED I OPERATION I I SCREEN EP SUPPLY INVENTOR. VOLTAGE CARL0s A. TOMASZEWSK/ "F'7H2 p 10) 74 ATTORNEY United States Patent3,273,009 YOKE DRIVING CIRCUIT WHICH REDUCES TRANSIENTS Carlos A.Tomaszewski, Canoga Park, Califi, assignor,

by mesne assignments, to Stromherg-Carlson Corporation, Rochester, N.Y.,a corporation of Delaware Filed Jan. 2, 1964, Ser. No. 335,194 Claims.(Cl. 315-27) The present invention relates to cathode-ray tubeelectromagnetic yoke driving circuitry.

Readout equipment for data processors may utilize shaped beamcathode-ray tubes. These tubes include a beam-forming matrix whichshapes the beam to form indicia prior to its striking the face of thetube at a particular point which is controlled by passing different D.C.currents through X and Y direction electromagnetic yokes. Such readoutequipment is disclosed in US. Patent 2,924,- 742, assigned to the sameassignee as the present invention.

In order to facilitate rapid information readout, it is important thatthe beam be shifted from one position to another in the shortestpossible time. However, abrupt current changes within theelectromagnetic yokes cause random oscillations, or transients, to begenerated within the yokes. As a result, the settling time of the beamupon being shifted from one position to another is considerably greaterthan it would be in the absence of these transients. In prior art yokedriving circuits, a triode was connected in series with a currentmonitoring resistance connected to the cathode of the triode and anelectromagnetic yoke coupled to the anode of the triode. Sharp increasesin plate current which were necessary to rapidly shift the beam alsocaused large Ldi/dt voltage drops across the yoke which, in turn,sharply reduced the positive voltage at the plate of the triode. As aresult of this drop in plate voltage, plate current flow and hencecurrent flow through the yoke was sharply reduced temporarily whichaggravated the production of transients and increased the settling timeof the beam. The fact that the plate current of a tetrode or pentode isrelatively independent of plate voltage suggested the use of such a tubein place of a triode. However, the screen current would pass through theaforementioned current monitoring resistance and an error would beproduced in the current monitoring circuit, since this circuit is onlysupposed to measure the current which passes through the yoke. Thismonitoring circuit may be used to compensate for parameter changes whichcould result in beam positioning errors in the absence of the monitoringcircuit, or for other purposes that have nothing to do with theinvention.

In accordance with the present invention, triode operation of a tetrodeor pentode is obtained by electrically coupling the screen grid to theplate during steady state operation by forward biasing a first diodeconnected between the plate and the screen grid. When the currentthrough the tube is sharply increased and a sharp drop in plate voltageis manifested due to the Ldi/dt drop across the yoke winding during thetransient period, the first diode becomes back biased to electricallydecouple the screen grid from the plate and a second diode connectedbetween the screen grid and a screen supply is now forward biased sothat the tube operates as a tetrode or pentode until the transientcondition is terminated. As a result, the plate current is held to arelatively high level during the transient period and no error due toscreen current is generated during steady state operation when such anerror is undesirable.

Accordingly, it is an object of the present invention to provide currentcontrol circuitry employing a single tetrode or pentode which combinescharacteristics of both triodes and tetrodes, or pentodes.

ICC

It is a further object of the present invention to provide new andimproved electromagnetic yoke driving circuitry which sharply reducesthe effects of transients set up within an electromagneticbeam-positioning yoke.

Further objects and advantages of the invention will become apparent asthe following description proceeds, and the features of novelty whichcharacterize the invention will be pointed out with particularity in theclaims annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had to theaccompanying drawings, in which:

FIG. 1 discloses a preferred embodiment of the invention; and

FIG. 2 discloses current-voltage curves which will be useful in theunderstanding of the operation of the embodiment of the inventiondisclosed in FIG. 1.

FIG. 1 schematically discloses a cathode-ray tube 1 having anelectromagnetic yoke 2 associated therewith. A pentode 3 is shownhaving. its cathode grounded through resistance 4 and having its anodeconnected to plate supply 6 through yoke 2. Input circuit 7 is coupledto the control grid so that various changes in the voltage impressedupon the control grid may be utilized to control the current throughyoke 2 which, in turn, controls the beam position within cathode-raytube 1. The screen grid of pentode 3 is coupled to screen supply 8through diode 9 and is also coupled to the plate via diode 11. Thesuppressor grid is directly connected to the cathode in the conventionalmanner.

Let it be assumed that the current through yoke 2 is to be sharplyincreased to shift the beam within cathoderay tube 1. Since theresistance of yoke 2 is relatively small, the voltage on the plate oftube 3 will be almost as positive as the plate supply voltage and,accordingly, diode 11 will be forward biased so that the screen grid iselectrically coupled to the plate. On the other hand, diode 9 will beback biased since the screen voltage is considerably less than the platevoltage. Some electrons will flow through the screen grid, diode 11 andyoke 2, together with the electrons which are gathered up by, the plate.Accordingly, the IR drop across current monitoring resistor 4 willaccurately reflect the yoke current. Now let it be assumed that thevoltage impressed upon the control grid is made more positive so as tosharply increase the plate current. This tends to cause the platevoltage to become sharply reduced which, in turn, would sharply reducethe plate current. In other words, the abrupt reduction in plate voltagewould cause a correspondingly abrupt drop in plate current during thetransient period, if tube 3 were operating as a triode. Diodes 9 and 11,however, are arranged so that when the plate voltage is reduced belowthe screen supply voltage, tube 3 will operate as a pentode which meansthat the plate current would not be substantially reduced during thetransient condition and therefore the transient will die out muchfaster. Under these circumstances, the reduction of the plate voltagebelow the screen supply voltage causes diode 11 to become back biased todecouple the screen grid from the plate. In addition, diode 9 becomesforward biased and the screen supply voltage is applied to the screengrid so that tube 3 operates as a pentode as long as the plate voltageof tube 3 is somewhat below the screen supply voltage. As the currentthrough yoke 2 approaches the higher steady state level, the Ldi/dt dropwill decrease and the plate of tube 3 will again become more positivethan the screen voltage so as to forward bias diode 11 and back biasdiode 9, and, accordingly, triode operation is resumed. The fact that acomponent of the current flowing through the monitoring resistance 4 isentering the screen supply through diode 9 during the transient orpentode operation period is of no importance. This is because theaforementioned error introduced by this component into currentmonitoring circuits has no effect during the transient period. Aftersteady state conditions are re-established, diode 9 becomes back biased,as mentioned hereinabove, triode operation is again manifested, and thevoltage produced across the current monitoring resistance 4 willaccurately reflect the current through yoke 2.

While there has been shown and described a specific embodiment of theinvention, other modifications will readily occur to those skilled inthe art. It is not, therefore, desired that this invention be limited tothe specific arrangement shown and described, and it is intended in theappended claims to cover all modifications within the spirit and scopeof the invention.

What is claimed is:

1. In a cathode-ray tube yoke driving circuit, a current control elementhaving an input electrode, an output electrode, a control electrode anda screen electrode; a first D.C. voltage source having a first andsecond terminal, a cathode-ray tube having a beam-positioningelectromagnetic yoke winding associated therewith, said yoke windinghaving a first and second terminal, means for coupling the inputelectrode of said current control element to the second terminal of saidfirst D.C. voltage source, means for coupling the output electrode ofsaid current control element to the first terminal of saidelectromagnetic yoke, means for coupling the output terminal of saidelectromagnetic yoke to the first terminal of said first D.C. voltagesource, a second D.C. voltage source for producing a screen voltagelower than the voltage produced by said first voltage source, an inputcircuit coupled to said control electrode for abruptly changing thecurrent flowing through said electromagnetic yoke winding and saidcurrent control element to alter the position of the beam within saidcathode-ray tube, means for effectively coupling said screen electrodeand said output electrode together when the voltage at the outputelectrode is greater than a predetermined voltage somewhat lower thanthe voltage produced by said second D.C. voltage source and foreffectively decoupling said output electrode and said screen electrodeas long as the voltage at the output electrode remains below saidpredetermined voltage, and means for effectively coupling said secondD.C. voltage source to said screen electrode only during that periodwhen the voltage at the output electrode remains below saidpredetermined voltage.

2. In combination, a current control element having an input electrode,an output electrode, a control electrode and a screen electrode; a firstD.C. voltage source, a reactance circuit, means for coupling said firstD.C. voltage sOurce, said reactance circuit, said input electrode andsaid output electrode in series to cause D.C. current to flow throughsaid reactance circuit; a second D.C. voltage source, an input circuitcoupled to said control electrode for abruptly changing the currentflowing through said reactance circuit and said current control element,means for effectively coupling said screen electrode and said outputelectrode together when the voltage at the output electrode is greaterthan a predetermined voltage somewhat lower than the voltage produced bysaid second D.C. voltage source and for effectively decoupling saidoutput electrode and said screen electrode as long as the voltage at theoutput electrode remains below said predetermined voltage, and means foreffectively coupling said second D.C. voltage source to said screenelectrode only during that period when the voltage at the outputelectrode remains below said predetermined voltage.

3. In a current control element having an input electrode, an outputelectrode, a control electrode and a screen electrode; a first D.C.voltage source having a first and second terminal, a reactance circuit,means for coupling the first terminal of said first D.C. voltage sourceto said output electrode through said reactance circuit,

means for coupling the second output terminal of said first D.C. voltagesource to said input electrode, a second D.C. voltage source, an inputcircuit coupled to said control electrode for abruptly changing thecurrent flowing through said reactance circuit and said current controlelement, means for effectively coupling said screen electrode and saidoutput electrode together when the voltage at the output electrode isgreater than a predetermined voltage somewhat lower than the voltageproduced by said second D.C. voltage source and for effectivelydecoupling said output electrode and said screen electrode as long asthe voltage at the output electrode remains below said predeterminedvoltage, and means for effectively coupling said second D.C. voltagesource to said screen electrode only during that period when the voltageat the output electrode remains below said predetermined voltage.

4. In combination, a multi-element electron tube having at least ananode, a cathode, a control grid and a screen grid; a cathode-ray tubehaving a beam-positioning electromagnetic yoke winding associatedtherewith, a power supply having a first terminal for supplying B+ tothe anode of said electron tube, a second terminal for applying apositive D.C. screen potential to the screen electrode of said electrontube and a third ground reference potential terminal, means for couplingthe cathode of said electron tube to the third terminal of said powersupply, means for coupling said yoke winding between the first terminalof said power supply and said anode, an input circuit coupled to thecontrol grid of said electron tube for abruptly increasing the currentflowing through said tube thereby to reduce the potential at the plateof said electron tube below the potential at the second terminal of saidpower supply, means for effectively coupling said screen grid to saidplate and for decoupling said screen grid from the second terminal ofsaid power supply as long as the potential at said anode is higher thana predetermined voltage somewhat lower than the potential at the secondoutput terminal of said power supply and for effectively decoupling thescreen grid from said plate and for causing the second terminal of saidpower supply to be effectively coupled to said screen grid as long asthe voltage at said anode remains below said predetermined voltage.

5. In combination, a multi-element electron tube having at least ananode, a cathode, a control grid and a screen grid; a reactance circuit,a power supply having a first terminal for supplying B+ to the anode ofsaid electron tube, a second terminal for applying a positive D.C.screen potential to the screen electrode of said electron tube and athird ground reference potential terminal, means for coupling thecathode of said electron tube to said third terminal of said powersupply, means for coupling said reactance circuit between the firstterminal of said power supply and said anode, an input circuit coupledto the control grid of said electron tube for abruptly increasing thecurrent flowing through said tube thereby to reduce the potential at theplate of said electron tube below the potential at the second terminalof said power supply, means for effectively coupling said screen grid tosaid plate and for decoupling said screen grid from the second terminalof said power supply as long as the potential at said anode is higherthan a predetermined voltage somewhat lower than the potential at thesecond output terminal of said power supply and for effectivelydecoupling the screen grid from said plate and for causing the secondterminal of said power supply to be effectively coupled to said screengrid as long as the voltage at said anode remains below saidpredetermined voltage.

No references cited.

DAVID G. REDINBAUGH, Primary Examiner. T. A. GALLAGHER, AssistantExaminer.

4. IN COMBINATION, A MULTI-ELEMENT ELECTRON TUBE HAVING AT LEAST ANANODE, A CATHODE, A CONTROL GRID AND A SCREEN GRID; A CATHODE-RAY TUBEHAVING A BEAM-POSITIONING ELECTROMAGNETIC YOKE WINDING ASSOCIATEDTHEREWITH, A POWER SUPPLY HAVING A FIRST TERMINAL FOR SUPPLYING B+ TOTHE ANODE OF SAID ELECTRON TUBE, A SECOND TERMINAL FOR APPLYING APOSITIVE D.C. SCREEN PORTENIAL TO THE SCREEN ELECTRODE OF SAID ELECTRONTUBE AND A THIRD GROUND REFERENCE POTENTIAL TERMINAL, MEANS FOR COUPLINGTHE CATHODE OF SAID ELECTRON TUBE TO THE THIRD TERMINAL OF SAID POWERSUPPLY, MEAND FOR COUPLING SAID YOKE WINDING BETWEEN THE FIRST TERMINALOF SAID POWER SUPPLY AND SAID ANODE, AN INPUT CIRCUIT COUPLED TO THECONTROL GRID OF SAID ELECTRON TUBE FOR ABRUPTLY INCREASING THE CURRENTFLOWING THROUGH SAID TUBE THEREBY TO REDUCE THE POTENTIAL AT THE PLATEOF SAID ELECTRON TUBE BELOW THE POTENTIAL AT THE SECOND TERMINAL OF SAIDPOWER SUPPLY, MEANS FOR EFFECTIVELY COUPLING SAID SCREEN GRID TO SAIDPLATE AND FOR DECOUPLING SAID SCREEN GRID FROM THE SECOND TERMINAL OFSAID POWER SUPPLY AS LONG AS THE POTENTIAL AT SAID ANODE IS HIGHER THANA PREDETERMINED VOLTAGE SOMEWHAT LOWER THAN THE POTENTIAL AT THE SECONDOUTPUT TERMINAL OF SAID POWER SUPPLY AND FOR EFFECTIVELY DECOUPLING THESCREEN GRID FROM SAID PLATE AND FOR CAUSING THE SECOND TERMINA OF SAIDPOWER TO BE EFFECTIVELY COUPLED TO SAID SCREEN GRID AS LONG AS THEVOLTAGE AT SAID ANODE REMAINS BELOW SAID PREDETERMINED VOLTAGE.